1. Field of the Invention
This invention generally relates to the field of polymers. More specifically, the invention relates to wellbore treatment compositions using hydrophilically modified polymers.
2. Background of the Invention
Associative thickening or gelation of water soluble polymers has many potential applications in such diverse fields as surface coatings, personal care products, drug delivery systems, and other areas requiring rheology modification or colloid stabilization. In particular, such materials have been used in oil extraction such as in oil-field well treatment, enhanced oil recovery, etc. One problem with current wellbore treatment fluids are the high concentrations of polymer required to achieve the necessary viscosity for oilfield applications.
Water soluble polymers modified with hydrophobic side chains, have been investigated with much higher viscosity than similar concentrations of unmodified polymers. This viscosity increase occurs due to association of the hydrophobic side chains, creating a network structure. Hydrophobic polymer modification is commonly achieved by adding alkyl side chains onto a water soluble base polymer. The hydrophobic side chains may also be fluorinated so as to increase hydrophobicity. The polymer modification is progressed via reaction with, for example, a halide, an acyl halide, an anhydride, an epoxide, an amine, or an isocyanate of the required hydrophobic ligand. The size and number of hydrophobic side chains that may be grafted is limited by the solubility of the modified polymer. Hydrophobic polymer modification results in lower solubility, which manifests itself in increase dissolution time or insolubility. The degree of hydrophobic modification is generally limited so as to ensure solubility. The viscosity of aqueous dispersions of polymers modified with hydrophobic side chains reaches a maximum after complete dissolution of the polymer. The viscosity of these solutions decreases as the temperature of the solution is increased. Natrosol Plus, Grade 430, supplied by Hercules is an example of a commercially available water soluble polymer modified with hydrophobic side chains. Hydrophobic modification can be applied to numerous water soluble polymers, including polyvinyl alcohol (PVA), polyacrylic acid (PAA), polymethacrylic acid (PMA), polyacrylamide (PAM), polyethylene oxide (PEO), polypropylene oxide (PPO), carboxymethyl cellulose (CMC), hydroxyethyl cellulose (HEC), hydroxypropyl cellulose (HPC), guar gum, hydroxypropyl guar gum (HPG), carboxymethylhydroxypropyl guar (CMHPG), dextran, locust bean gum (LBG), welan gum, xanthan gum, scleroglucan, succinoglycan, polypeptides and the like. Nonionic polymers, or predominantly nonionic polymers, generally have greater compatibility with other species in aqueous solution and have broader applicability.
When combined with suitable surfactants, water soluble polymers modified with hydrophobic side chains can provide solutions with much higher viscosity than similar concentrations of polymers modified with hydrophobic side chains alone. The lower concentration of modified polymer generally results in improved solubility, but solubility still limits the degree of hydrophobic modification of the polymer that can be applied. Many anionic surfactants will interact with polymers modified with hydrophobic side chains, in aqueous solution. As an example, sodium dodecyl sulfate (SDS) is a common surfactant that demonstrates interaction in solution with water soluble polymers modified with hydrophobic side chains. A large range of cationic surfactants also associate in solution with polymers modified with hydrophobic side chains.
Synthetic water soluble copolymers that incorporate hydrophobic alkyl moieties, in conjunction with nonionic surfactants have been described. For example, copolymers of acrylamide and dodecylacrylate, along with one or more nonionic surfactants and a mono-valent inorganic salt have been reported. The performance of these essentially ionic synthetic polymers (the nature of which is not essentially changed after incorporation of hydrophobic alkyl moieties) is strongly influenced by water salinity. When no salt is added, the viscosity generation is poor. The ionic character of the polymer backbone can also increase incompatibility with other water soluble species, such as divalent, trivalent or polyvalent ions. This can result in a reduced efficacy, or ultimately precipitation from solution, of the associative copolymer.
The degree of hydrophobic modification of water soluble polymers can be increased, while retaining solubility, by also adding a hydrophilic modification to the same polymer. Hydrophobic-hydrophilic polymer modification has been achieved by grafting ionic side chains onto a water soluble base polymer, along with hydrophobic side chains. The grafting of sulfoalkyl groups along with alkyl groups has been described. The sulfoalkyl group may be added by reaction of a water soluble polymer containing pendant hydroxyl groups with, for example, 3-chloro-2-hydroxy-propane sulfonate, sodium 3-bromopropane sulfonate or sodium vinylsulfonate and the like. For synthetic polymers, the hydrophobic-hydrophilic polymer can be formed by the copolymerization of suitable monomers, at least one of which provides ionic character for the resulting polymer and at least one of which provides hydrophobic character for the resulting polymer. An example of such a co-polymerization is the reaction of an acrylamide with an alkyl methacrylate, forming a poly(co-acrylamide-alkylmethacrylate). Although ionic polymers, or ionically modified polymers, can increase solubility in water, such ionic character can also increase incompatibility with other water soluble species, such as divalent, trivalent or polyvalent ions. This can result in a reduced efficacy, or ultimately precipitation from solution, of the modified associative polymer.
Viscoelastic surfactants are another class of associative materials that have been taught. The viscoelastic surfactant molecules, when present at a sufficiently high concentration, aggregate into micelles, which may take the form of rods or worm-like micelles, resulting in an associative structure that provides an increase in viscosity. Many surfactants may be used to form viscoelastic solutions, for example, N-erucyl-N,N-bis(2-hydroxyethyl)-N-methyl ammonium chloride is a commercially available viscoelastic surfactant. The ionic strength of the solution of viscoelastic surfactants is selected so as to improve viscosity generation. For low salinity water, this generally requires the addition of one or more mono-valent halides or salts of organic anions, with the cation being selected from sodium, potassium or ammonium or the like. Even with an adjusted ionic strength, a high concentration surfactant solution is still required in order to provide a significant increase in viscosity. A surfactant concentration in excess of 5% by weight is not uncommon, and even at this concentration the viscosity of systems at elevated temperature is somewhat limiting. The high surfactant concentration also makes viscoelastic surfactant systems commercially unattractive for many applications. The ionic character of the viscoelastic surfactant can also increase incompatibility with other water soluble species, such as divalent, trivalent or polyvalent ions. This can result in a reduced efficacy of the viscoelastic surfactant system.
The use of a viscoelastic surfactant has been combined with a water soluble polymer modified with hydrophobic side chains. The pendant hydrophobic chains interact with the surfactant micelles creating a viscoelastic gel structure. This association occurs below the typical concentration used for pure visco-elastic surfactant systems, thus providing the potential for more commercially viable applications of visco-elastic surfactants. The solubility of the alkyl modified water soluble polymer limits the size and number of hydrophobic chains that may be incorporated into the polymer. Due to the ionic nature of the surfactant incorporated into the system, incompatibility with other water soluble species, such as divalent, trivalent or polyvalent ions can occur. This can result in a reduced efficacy of the system.
Gelation or associative thickening can also be achieved with water soluble polymers containing hydrophilic groups. The use of visco-elastic surfactants combined with a water soluble polymer that, after dissolution and heating to the lower critical solution temperature (LCST) of the polymer, provides a substantial increase in viscosity. For example, a solution of a block-copolymer of PEO and PPO linked by urethane, urea and allophanate bonding units and the visco-elastic surfactant N-erucyl-N,N-bis(2-hydroxyethyl)-N-methyl ammonium chloride that is heated to a temperature greater than the LCST of the polymer, provides higher viscosity than similar concentrations of the same polymer heated to the same temperature when no surfactant is present. Due to the cationic nature of the surfactant incorporated into the system, incompatibility with other water soluble species, such as divalent, trivalent or polyvalent ions can occur. This can result in a reduced efficacy of the system.
The use of a charged polymer along with a surfactant of an opposing charge can also be used to create association in solution. As an example, when a negatively charged ionic polymer, such as CMC, CMHPG, PAA, PMA or the like is mixed in solution with a positively charged surfactant, such as a tetraalkylammonium halide or the like, at suitable concentrations of the components, association is observed via an increase in viscosity compared to similar concentrations of ionic polymer without the addition of surfactant. Similarly, as a further example, when a positively charged ionic polymer, such as poly(2-(dimethylamino)ethyl methacrylate or N-[3-(dimethylamino)propyl]methacrylamide, or a cellulosic water soluble polymer modified by the addition of a quaternary ammonium group or the like, is mixed in solution with a negatively charged surfactant, such as an sodium dodecyl sulfate, or the like, at suitable concentrations of the components, association is observed via an increase in viscosity compared to similar concentrations of ionic polymer without the addition of surfactant. Although these charged polymer-ionic surfactant systems can provide associative thickening, such ionic character increases the incompatibility of these systems with other water soluble species, such as divalent, trivalent or polyvalent ions. This can result in a greatly reduced efficacy of such systems, and ultimately polymer precipitation from solution.
In addition, present wellbore treatments fluids often require the use of separate “breaking” compounds to reduce viscosity after the wellbore treatment fluid's function has been completed such as in a fracture fluid. A disadvantage of existing wellbore treatment fluids, is the incomplete breaking of the viscous wellbore treatment fluid as the addition of breaking compounds are not able to completely break the chemical crosslinks in the wellbore treatment fluids. Thus, the “broken” wellbore treatment fluid still has a relatively high viscosity even after breaking.
Consequently, there is a need for wellbore treatment compositions with improved efficacy (i.e. lower polymer concentration, more effective breaking, elimination of precipitation, etc.).